The photochlorination reactions use photons to initiate chlorination of pendant free radicals. In some reactions, a free radical photoinitiator is used alone or in combination with a free radical thermal initiator to form a composite initiation system. Sometimes, a second component and even a third component may be added to prevent chlorine substitution on the benzene ring as side-reaction. Generally, a mercury lamp is used as a light source. Because purity of trichloromethyl-substituted benzene is not ideal, there is great difficulty using this technique to realize industrial mass production.
The present inventors have recognized that the method of preparing trichloromethyl-substituted benzene via photochlorination in the prior art has drawbacks in the following ways.
1) The photochlorination reaction is a radical chain reaction and due to the side-reaction, chlorination site and chlorination depth are relatively difficult to control. In order to separate complex photochlorination products, a great number of rectification operations have to be performed in DE3146868 and JP 57-130931, which greatly increases the production cost of such products. To prevent chlorine substitution on the benzene ring, sulfur and acetyl chloride are added in U.S. Pat. No. 1,345,373, a metal carbonate is added in U.S. Pat. No. 1,384,909, phosphor and sulfur are added in U.S. Pat. No. 1,733,268, an organic base is added in U.S. Pat. No. 2,034,962, an amide is added in U.S. Pat. No. 2,695,873, amines are used in U.S. Pat. No. 2,817,632 and U.S. Pat. No. 2,844,635, and triphenyl phosphine is used in U.S. Pat. No. 3,363,013. These additional components unavoidably affect the purity and subsequent purification of trichloromethyl-substituted benzene. It is reported in U.S. Pat. No. 4,029,560 and U.S. Pat. No. 4,048,033 that in the chlorination, the target product is used as solvent to inhibit chlorine substitution on the benzene ring as side-reaction, and for example, in the chlorination of 1,3-dimethylbenzene, 1,3-bis-(trichloromethyl)-benzene is used as solvent, which requires a great amount of 1,3-bis-(trichloromethyl)-benzene to be used repeatedly. Thus, this method has complex process and high cost.
In summary, in order to achieve chlorination of all hydrogen atoms on pendant methyl groups without chlorination of hydrogen atoms on the benzene ring in the prior art, multiple adjuvant components need to be introduced, which will ‘contaminate’ the target product trichloromethyl-substituted benzene and thus are not suitable for preparation of high-purity products.
2) A free radical initiator is also required to initiate the photochlorination reaction.
Wang Lumin et al. (Journal of Tonghua Normal University, 2005, 26(4):46-47) have found that a free radical initiator is required to maintain the reaction for the photochlorination of 1,3-dimethylbenzene.
A method of preparing tetrachloro-o-xylene from o-xylene via photochlorination in three temperature stages is disclosed in CN102211975A. In this method, the photochlorination includes three temperature stages of 120-125° C., 125-130° C. and 130-135° C., which correspond to the amounts of chlorine introduced of ⅓, ½ and ⅙ of the total amount of chlorine, respectively. Similarly, benzoyl peroxide is added as a light sensitive catalyst in this reaction. After this reaction in the three temperature stages is completed, the yield of tetrachloro-o-xylene is only 65% and the yield of pentachloro-o-xylene is 10%. Because of the addition of the light sensitive catalyst in this reaction, the purity of the resulting tetrachloro-o-xylene only reaches 90% even in case of further purification.
3) A mercury lamp is generally used as a light source in a photochlorination reaction. However, the said light source has numerous disadvantages.
The present inventors have found that the short-wavelength light of a low-pressure mercury lamp can bring out other photochemical side reactions, resulting in decreased product purity, and the long wavelength light of a high-pressure or medium-pressure mercury lamp is not sufficient to give rise to a chlorine radical reaction, resulting in increased energy consumption. In addition, more heat is generated when a mercury lamp is used as a light source; and thus it is necessary to provide a corresponding cooling device, making the reactor structure complicated.
It is disclosed in CN1948245 that a light emitting diode (LED) having a wavelength range of 300-600 nm and a power range of 0.1 W-1000 W is used as a light source in a photochlorination reaction to produce benzyl chloride, where the reaction temperature is maintained at 90-150° C. It is recorded in the document that its technical problem to be solved is to provide a photochlorination method with low power consumption and low heat generation from the light source; and the utilization rate of the light source can be improved by selecting the light emitting diode as the light source. Although this document mentioned that m-dimethylbenzene may be used as a raw material, all the examples of this document do not disclose the purity and the yield of the product.
The applicant has also found that the illuminance of the light source for this reaction is not researched by the prior art.
In addition, bis-(trichloromethyl)-benzene in trichloromethyl-substituted benzene can react with water or phthalic acid to prepare an intermediate of aramid fiber, bis-(chloroformyl)-benzene. For producing aramid fiber, a high purity of bis-(chloroformyl)-benzene is needed as a starting material, otherwise the quality of aramid fiber is difficult to meet the specified requirements. Further, relevant research on the purification of bis-(trichloromethyl)-benzene has been performed by the applicant. In conventional processes, such as distillation and rectification under atmospheric pressure, separation and purification are achieved depending on the different boiling points of compounds, and it is required to remain in a high-temperature environment for a long time. In this case, partial polymerization will be generated. Thus, use of such purification processes leads to coke formation, causing damage to the apparatus which then needs to be periodically cleaned. On the other hand, the coke is harmful to the environment and needs to be properly handled, resulting in high environmental cost. For vacuum rectification, although the temperature required for separation can be reduced, the material to be separated must be maintained at a certain level in a re-boiler to generate a static pressure difference, so that the vaporizing temperature of the material in a column reactor is increased, and thus thermal decomposition of the material may be difficult to avoid in some cases. The presence of inert gases is beneficial to rectification of the heat sensitive material, but it causes problems in condensation or cooling. For recrystallization process, consumption of a substantial amount of solvent is required, which causes pollution to the environment, and impurity carried by the solvent contaminates the product.
Among the preparation methods of bis-(chloroformyl)-benzene in the prior art, the thionyl chloride method with phthalic acid as a raw material is most commonly used (for example, see CN 102516060A, CN 102344362A). However, in the process, phthalic acid having a high purity of 99.99% is required to obtain desired bis-(chloroformyl)-benzene, which results in a significant increase in the preparation cost and is a more difficult process.
In addition, relevant research on the apparatuses for photochlorination reaction has been performed by the applicant. Photochlorination reactors are widely used in the field of chemical production. Most of the existing apparatuses for photochlorination reaction compose three parts, a reactor, a light source, and jacketed condenser. For example, the photochlorination reactors disclosed in patents CN200942338Y and CN101456788B are essentially equivalent and both include a cooling jacket outside a cylinder, an anti-corrosion material lining the cylinder, a sprayer, and light sources arranged at angles. However, in the two photochlorination reactors, desired increase in illumination intensity and range is not achieved, and uneven illumination distribution exists in the reactors, which easily causes side reactions in the photochlorination. In addition, in the disclosed technological solutions, both ends of tubes in which the light sources are placed extends through the reactor cylinder; so that in actual production process, when the reaction temperature is higher, uneven heating of the tubes may easily be caused, resulting in damage to the tubes.